Texture Paint not Appearing on Model
Without seeing the blend file I cannot be certain that this is the cause of your issue, but I have had this exact problem before when I was painting on a texture imported from a .dds (DirectDraw Surface) file.
When re-saved as a png file Blender could draw to it in memory and it updated just fine on the model.  When left as .dds it just wouldn't work, or would update in odd infrequent spots.  Even though the file is held in memory in Blender and it shouldn't matter what format it's in on disk... it does.  If you have a texture file and it isn't png or something equally simple, try saving it as that?